High Availability, or HA as it is abbreviated, refers to the availability of resources in a computer system, in the wake of component failures in the system. This can be achieved in a variety of ways, spanning the entire spectrum ranging at the one end from solutions that use custom and redundant hardware to ensure availability, to the other end to solutions that provide software solutions using off-the-shelf hardware components. The former class of solutions provide a higher degree of availability, but are significantly more expensive, than the latter class. This has led to the popularity of the latter class, with almost all vendors of computer systems offering various HA products. Typically, these products survive single points of failure in the system. [Source: IEEE Task Force on Cluster Computing]
